Move Timer.h and dependencies into blink_platform.dll

This CL required splitting PlatformThreadData off from ThreadGlobalData because
ThreadGlobalData depends on non-platform concepts.

R=jochen@chromium.org
BUG=297477

Review URL: https://codereview.chromium.org/25722016

git-svn-id: svn://svn.chromium.org/blink/trunk@158937 bbb929c8-8fbe-4397-9dbb-9b2b20218538